Shaft seal

ABSTRACT

A shaft seal is disclosed comprising a sealing element ( 2 ), a rotary sealing part ( 4 ) mounted coaxially with the sealing element and forming therewith a contactless primary seal between opposed faces of the sealing element ( 2 ) and rotary sealing part ( 4 ) to substantially prevent fluid flow across the primary seal from a high-pressure radial side to a low-pressure radial side, a seal housing ( 8 ), a pusher sleeve ( 9 ) disposed, between the seal housing and the sealing element, coaxially with and in contact with the sealing element ( 2 ), biasing means ( 10 ) urging the pusher sleeve ( 9 ) away from the seal housing ( 8 ) and against the sealing element ( 2 ) to urge the sealing element axially towards the rotary sealing member ( 14 ), and a first sealing member ( 12 ) disposed about the pusher sleeve ( 9 ) and located, in a channel ( 14 ), in communication with the high-pressure radial side to provide a secondary seal for the pusher sleeve ( 9 ) between the high-pressure and low-pressure radial sides. An auxiliary sleeve ( 20 ) is disposed around the pusher sleeve ( 9 ) coaxially therewith and maintained in sealing contact with the pusher sleeve ( 9 ) by the first sealing member ( 12 ). A tertiary seal ( 16 ) is provided in communication with the high-pressure radial side and formed between transverse and faces of the auxiliary sleeve ( 20 ) and seal housing ( 8 ). The auxiliary sleeve ( 20 ) becomes deformed under high operating fluid pressures in a manner conforming with that of the pusher sleeve ( 9 ), thereby reducing the risk of the sealing member ( 12 ) being blown out.

[0001] The invention relates to a shaft seal for rotating shafts inturbo-machines or other pressurized machine. In particular, the presentinvention, in common with WO-A-96/33357 provides a shaft seal comprisinga sealing element, a rotary sealing part mounted coaxially with thesealing element and forming therewith a contactless primary seal betweenopposed faces of the sealing element and rotary sealing part tosubstantially prevent fluid flow across the primary seal from a highpressure radial side to a low-pressure radial side, a seal housing, apusher sleeve disposed, between the seal housing and the sealingelement, coaxially with and in contact with the sealing element, biasingmeans urging the pusher sleeve away from the seal housing and againstthe sealing element to urge the sealing element axially towards therotary sealing part, and a first sealing member disposed about thepusher sleeve and located, in a channel, in communication with thehigh-pressure radial side to provide a secondary seal for the pushersleeve between the high-pressure and low-pressure radial sides.

[0002] Such a shaft seal is disclosed in WO-A-96/33357.

[0003] Non-contacting shaft seals are often used with machinery for thecompression or expansion of gas (hydrogen, natural gas, air, etc.) wherethe transmission of gas along the shaft needs to be prevented. Due tothe high-pressure, high-speed machinery which is normally used, theshaft seals are chosen to be of non-contact type, in order to reduceheat build up in the seals and the wear of the sealing parts and/or inorder to avoid the complexity of oil seals and their associated systems.

[0004] Non-contacting operation avoids this undesirable face contactwhen the shaft is rotating above a certain minimum speed, which is oftencalled a lift-off speed.

[0005] Non-contacting shaft seals provide advantages over seals wherethe sealing surfaces contact one another, due to reduction in wear andthe lower heat generation. Articles entitled “Fundamentals of SpiralGroove Non-contacting Face Seals” by (Gabriel, Ralph P. (Journal ofAmerican Society of Lubrication Engineers Volume 35, 7, pages 367-375),and “Improved Performance of Film-Riding Gas Seals Through Enhancementof Hydrodynamic Effects” by Sedy, Joseph (Transaction of the AmericanSociety of Lubrication Engineers, Volume 23, 1 pages 35-44) describenon-contacting seal technology and design criteria and are incorporatedherein by reference.

[0006] As with ordinary mechanical seals, a non-contacting face sealconsists of two principal sealing elements. At least one of the sealingelements is provided with shallow surface recesses.

[0007] These recesses are taper-shaped perpendicular to and concentricwith the axis of rotation, the tapering being in the direction oppositeto the direction of rotation of the shaft. In known contactless faceseals, both sealing elements, in the form of rings, are positionedadjacent to each other with the sealing surfaces in contact atconditions of zero pressure differential and zero speed of rotation. Oneof the rings is normally fixed to the rotatable shaft by means of ashaft sleeve, the other being located within the seal housing structureand allowed to move axially. The shaft seal is designed to enable axialmovement of the sealing ring and yet prevent or substantially minimizeleakage of the sealed fluid. For this reason, a sealing member is placedbetween the ring and the housing.

[0008] As mentioned above, to achieve non-contacting operation of theseal, one of the two sealing surfaces is provided with shallow surfacerecesses, which act to generate pressure fields that force the twosealing surfaces apart. When the magnitude of the forces resulting fromthese pressure fields is large enough to overcome the forces that urgethe seal faces closed, the sealing surfaces will separate and form aclearance, resulting in non-contacting operation.

[0009] As explained in detail in the above-referenced articles, thecharacter of the separation forces is such that their magnitudedecreases with the increase of face separation. Opposing or closingforces, on the other hand, depend on sealed pressure level and as suchare independent of face separation. They result from the sealed pressureand the spring force acting on the back surface of the axially movablesealing ring. Since the separation or opening force depends on theseparation distance between sealing surfaces, during the operation ofthe seal or on imposition of sufficient pressure, differentialequilibrium separation between both surfaces will establish itself. Thisoccurs when closing and opening forces are in equilibrium and equal toeach other. Equilibrium separation constantly changes within the rangeof gaps. The goal is to have the low limit of this range above zero.Another goal is to make this range as narrow as possible, because on itshigh end the separation between the faces will lead to increased sealleakage. Since non-contacting seals operate by definition with aclearance between sealing surfaces, their leakage will be higher thanthat of a contacting seal of similar geometry. Yet, the absence ofcontact will mean zero wear on the sealing surfaces and therefore arelatively low amount of heat generated between them. It is this lowgenerated heat and lack of wear that enables the application ofnon-contacting seals to high-speed turbo machinery and other pressuremachines, where the sealed fluid is gas. Turbo compressors are used tocompress this fluid and since gas has a relatively low mass, theynormally operate at very high speeds and with a number of compressionstages in series.

[0010] As explained in the above-referenced articles, the effectivenessof the seal is largely dependent upon the so-called balance diameter ofthe seal. This is also true for contact seals.

[0011] When pressure is applied from the outside diameter of the seal,reduction of the balance diameter results in a greater force pushing thetwo sealing faces together and so a smaller gap between the faces. Thus,less gas is leaked from the system.

[0012] Known compressors have been used for compressing gas at inletpressures of some 200 bar to delivery pressures of some 500 bar.Contactless shaft seals of the kind described above are typically usedto seal against the compressor inlet pressure. The trend in compressorrequirements nowadays is towards higher inlet and delivery pressures.However, such pressure levels give rise to a problem with thecontactless shaft seals described above, as is now explained withreference to FIGS. 1, 1a and 2, 2 a.

[0013]FIG. 1 is a partial longitudinal sectional view through the shaftseal showing the relevant structural elements of a non-contacting shaftseal of the type described above. The shaft seal is incorporated in aturbo-machine (not shown) such as a compressor in this example. There isshown a shaft seal 1 having a (non-rotating) sealing element or ring 2mounted coaxially with the shaft axis (denoted by reference numeral 3),and a rotary sealing part or ring 4 located coaxially with the sealingring 2, and therefore also with the shaft axis 3. It will he appreciatedthat the vertical sectional view of FIG. 1, for simplicity, shows onlythe portion of the shaft seal located above the shaft axis. The sealingring 4 is mounted on an inner sleeve 5 having a radial flange 5 aagainst which the sealing ring 4 abuts, the sleeve 5 being mounted onthe shaft 6 such that the shaft 6, inner sleeve 5 and rotary sealingring 4 co-rotate as a single rotary element. In addition, a locatingsleeve 7 is bolted to inner sleeve 5. The assembly comprising components4, 5 and 7 is prevented from displacement in one axial direction by alocating ring 21 and in the opposite axial direction by the highpressure acting inside the compressor.

[0014] The shaft seal also has a seal housing 8 and a pusher sleeve 9disposed between a radially inward flange 8 b of the seal housing 8 andsealing ring 2. The pusher sleeve has a radial flange 9 b against whicha plurality of biasing springs (one of which, 10, is shown in FIG. 1,)located at the same axial position in respective blind holes 11 inradially inward flange 8 b and distributed about the shaft axis, act tourge the pusher sleeve 9 against the sealing ring 2. The (non-rotary)sealing ring 2 and rotary sealing ring 4 together form a contactlessprimary seal when the turbo-machine (or pressurized machine) is inoperation, which substantially prevents fluid flow between the sealingfaces of the primary seal, from the high pressure radially outer side tothe low pressure radially inner side. The sealing face of sealing ring 2has shallow grooves cut into its front surface to generate the requiredseparation between the sealing faces of sealing rings 2, 4.Alternatively, the grooves could be formed in the rotary sealing ring 4.

[0015] Preferred designs for the grooves are given in more detail inPublish International Application WO-A-96/15397 of Dresser-Rand Companyand the preferred designs for the groove are incorporated herein byreference. The sealing element 2 is normally made from carbon or othersuitable material.

[0016] As shown in FIG. 1, the sealing element 2 is afforded limitedaxial movement against the biasing force of the springs 10. Thesesprings provide a relatively small net biasing force so that when theshaft is rotating at normal speed, the generated separating forces causethe sealing ring 4 to separate from the sealing ring 2. The gap betweenthese rings adjusts itself such that the generated opening forces on theone hand and the sum of the generated closing forces and the springbiasing force on the other hand are equal to one another. However, wherethe shaft is at rest the springs act to move the sealing ring 2 intocontact with the rotary sealing ring 4.

[0017] A high-pressure gas is supplied to the radially outer is edge ofthe seal rings 2, 4. Normally, this gas would be derived from theworking fluid of the machine. However, it could instead be a clean gassuitable for venting into the atmosphere. In that event, the vented gascan be a combustible gas which is piped to burn (flare).

[0018] The high pressure at the high-pressure radial side acts aroundthe rear face of sealing element 2 down to a so-called equilibriumbalance diameter. Secondary seals 12, 13 are provided to prevent thehigh pressure venting around the rear face of sealing element 2 to thelow-pressure radial side (atmospheric pressure). The balance diameter isdetermined essentially by the contact line of secondary seal 12 with thehousing 8.

[0019] The first secondary seal 12 is provided between the pusher sleeve9 and the radially inward flange 8 b of the seal housing B. This sealcan be of any suitable form, such as a conventional O-ring, or, asshown, a spring-energised U-seal. Other forms of seal are possible andthe precise form selected is not material. The first secondary seal 12,as shown in FIGS. 1, 1a, is located in a channel 14 formed in the mainaxially-extending sleeve portion 9 a of the pusher sleeve 9. Thissecondary seal presses sealingly against the bottom of the channel 14formed in the main axially-extending sleeve portion 9 a. It also pressessealingly against the axially-extending inner radial face of theradially inward flange 8 b, thereby defining the equilibrium balancediameter for the shaft seal when operating in its equilibrium mode.

[0020] The further secondary seal 13 is provided between the rear faceof sealing ring 2 and the radial flange 9 b of pusher sleeve 9. Again,this secondary seal can take the form of an O-ring or, as shown, aspring-energised U-seal or Y-seal. The secondary seal 13 is located in achannel 15 formed in pusher sleeve 9. Alternatively, the channel 15could be formed in sealing element 2.

[0021] In use of the shaft seal 1, the high-pressure working fluid ofthe compressor is admitted to the high-pressure radial side of theprimary seal. This pressure acts on an outer annular region of the frontface of the radial flange 9 a of pusher sleeve 9, the outer annularregion having an inner diameter defined by the line of sealing of thesecondary seal 13 against the sealing ring 2 and the radially outerdiameter of radial flange 9 a. The high-pressure fluid also acts againstthe rear face of pusher sleeve 9 and down to the balance diameter. Thesecondary seals 12, 13 seal the applied high-pressure from thelow-pressure radial side, which is at atmospheric pressure where asingle shaft seal is used or, if multiple shaft seals are provided incascade, at a lower pressure than the pressure to be sealed. Because ofthe pressure differential acting on the area of the rear face of radialflange 9 a from the sealing diameter of the seal 13 down to the balancediameter, there is a net closing force (to the left in FIG. 1) acting onthe pusher sleeve 9, against the sealing ring 2 at all times. Thisclosing force is supplemented by the action of the biasing springs 10,and these closing forces are applied in the closing direction againstsealing ring 2. In addition, the high pressure fluid acting on the frontfaces of sealing ring 2 produces an opening force, while the highpressure fluid acting on the rear faces down to the sealing diameter ofsecondary seal 13 produces a closing force. Still further, thetaper-shaped surface recesses or grooves cut in the front face ofsealing ring 2 (or rear face of sealing ring 4) generate separatingpressure fields acting between the sealing rings 2, 4, the magnitude ofthe pressure fields depending on the rotational speed of the compressorshaft. The high pressure to be sealed, the depths of the recesses orgrooves and the size of the gap between the sealing rings 2, 4 alsoinfluence the magnitude of the pressure fields. Whether the sealingrings 2, 4 of the shaft seal are in contact or separated depends on themagnitudes of the generated opening and closing forces, and the netspring biasing force.

[0022] When the compressor is started tap, as the rotational speed ofthe shaft 6 initially starts to build up, the primary seal maintains asubstantially fluid-tight seal between the high-pressure andlow-pressure radial sides, by virtue of sealing contact between thesealing rings 2, 4. Under these conditions, the net separating forcegenerated by the primary seal is insufficient to overcome the sum of thespring biasing forces and the net closing force acting on the primaryseal due to the applied high-pressure.

[0023] However, when the compressor shaft speed reaches a sufficientvalue such that the applied fluid pressure is adequate to generate aseparating force that overcomes the net closing force acting on thesealing ring 2, this sealing ring will start to move away from thesealing ring 4 into an equilibrium position in which it maintains acontactless seal between the rotating sealing ring 2 and thenon-rotating sealing ring 4. As described above, the secondary seals 12,13 function at all times to prevent leakage of high-pressure fluid pastthe rear face of sealing ring 2 and the pusher sleeve 9.

[0024] An example of a shaft seal as described above is disclosed inWO-A-96/3335B, belonging to the present Applicant's affiliated CompanyDresser-Rand Company.

[0025] According to a known modification shown in FIG. 2, the channel14, instead of being formed in the pusher sleeve 9, is formed in theseal housing 8. Such arrangements (but only as such) are disclosed inEP-A-0591586 of Nippon Pillar Packing Co. Ltd. and U.S. Pat. No.5,421,593 belonging to the same proprietor.

[0026] Shaft seals of the type described above with reference to FIGS. 1and 2 operate satisfactorily at typical sealing pressures of compressorsthat have been manufactured in the past. Typically, such compressorshave been manufactured for compressing gases at pressures at typicallyfrom about 200 bar to about 500 bar. However, the industry is nowdemanding compressors to compress gas from 300 bar or more to 800 bar ormore. On the other hand, it has been found that existing shaft sealdesigns are not adequate to withstand such inlet-pressure values, forthe reasons now to be described with reference to FIGS. 1a and 2 a.

[0027] These Figures show, in deliberately exaggerated manner for thepurposes of illustration, the effect of operating under suchhigh-pressure values. As shown in these Figures, the high-pressureacting on the outer face of the axially-extending main sleeve portion 9a of the pusher sleeve 9 between the seal 12 and the junction with therear face of the radial portion 9 b deforms the main axially-extendingportion inwardly with a deflection increasing with increasing axialdistance in the axial direction away from the junction between the mainaxially-extending portion and the radial flange 9 a. This torsionaldeformation is indicated by letter A in FIG. 1a. Correspondingly, thehigh pressure acting against the inside (front) face of radially innerflange 8 b torsionally deforms that flange rearwardly, as indicated byarrow B. The consequence is that, as shown in FIGS. 1a, 2 a, the verysmall gap normally existing between the inner face of the radiallyinward flange 8 b of the seal housing 8 and the outer face of the mainaxially-extending sleeve portion 9 a of the pusher sleeve 9 is enlarged.With increasing high-pressure acting against the secondary seal 12 andwidening the gap between the flange 8 b and main sleeve portion of thepusher sleeve 9, a bead 12 b starts to form as the secondary seal 12starts to be extruded through the widening gap. When there is no suchbead on the secondary seal 12, this seal offers little frictionalresistance to the rearward axial sliding of the pusher sleeve 9.However, when the bead 12 b starts to form, the frictional resistanceincreases, potentially significantly and even to the point where thepusher sleeve can become united with the housing 8. Furthermore, as thebead 12 b continues to growl an increasingly unstable situation candevelop whereby the sealing ability of the secondary seal 12 isprogressively lessened due to the continuing extrusion, until eventuallyan unstable situation is reached in which the seal 12 is expelled orblown out through the gap, resulting in failure of the shaft seal. It isnoted that the bead 12 b does not normally form around the entire rearcircumferential region of the secondary seal 12 but generally only at asingle angular position about the seal circumference.

[0028] One possible solution to this problem that has been considered isto minimise the gap existing between the radial flange 8 b and thepusher sleeve 9 when the shaft seal is nor in use, but there is a limitto how much this gap can be reduced because the pusher sleeve 9 must befree to undergo limited axial movement when the shaft seal is not inoperation. Furthermore, radially inward deflection of the main sleeveportion of the pusher sleeve 9 is inevitable, yet this sleeve must notbe allowed to come into contact with the (rotating) shaft inner sleeve 5under full operating pressure.

[0029] Another potential solution which has been considered is to useharder materials for forming the sealing parts of the secondary seal 12,or to use back-up rings of harder material than the sealing partsthemselves of the secondary seal. However, there is a limit to how hardthe selected materials can be, particularly since harder materials areless effective to provide the required sealing effect and they alsoincrease the friction forces generated.

[0030] Spring energised polymer seals have been proposed. However, theoperating pressure at which beads start to form on such seals is about200-250 bar.

[0031] The present invention seeks to provide a shaft seal which isimproved in the above respects and can withstand high operatingpressures, in the range from zero bar to 300 bar or more. It relates toa shaft seal as initially defined and is characterised by an auxiliarysleeve disposed around the pusher sleeve coaxially therewith andmaintained in sealing contact with the pusher sleeve by the firstsealing member, the auxiliary sleeve being arranged to be urged in anaxial direction by fluid pressure acting at the high pressure radialside to form a tertiary seal.

[0032] Because the fluid high-pressure acting on the auxiliary sleeveproduces a net radially inwards force, it can be arranged that the smallgap existing between the pusher and auxiliary sleeves when no fluidpressure is applied to the shaft seal will not enlarge to the extentthat occurs in the prior art shaft seals disclosed with reference toFIGS. 1, 1a and 2, 2 a. Therefore, there is a reduced tendency forappreciable frictional resistance to develop between the first sealingmember and the seal housing, or for the first sealing member to beexpelled under high-pressure operation.

[0033] The function of the tertiary seal is merely to maintain sealingcontact between the seal housing and auxiliary sleeve. Ideally, thegeometry, material and design of the auxiliary sleeve is such that thedistortion of the auxiliary sleeve substantially matches that of thepusher sleeve under fluid pressure, so that the gap between these twoelements remains substantially the same irrespective of the fluidpressure acting, thereby avoiding or minimising the risk of a beadforming on the first sealing member.

[0034] Preferably, said channel in which said first sealing member islocated is formed in the pusher sleeve. This maximises the closing forceacting on the pusher sleeve when operating under high fluid pressure,because then the seal provided by the first sealing member against thepusher sleeve is located at a radially inward location. Alternatively,said channel in which said first sealing member is located may be formedin the auxiliary sleeve.

[0035] Preferably, the biasing means acts between the pusher sleeve andthe auxiliary sleeve. This arrangement guarantees that the sealingeffect of the tertiary seal is kept integral at all times, so that whenthe fluid pressure is applied it is prevented from passingunrestrictedly past the auxiliary sleeve to the low-pressure radialside. However, it is alternatively possible for the biasing means to actbetween the seal housing and the pusher sleeve, because it is consideredthat when the high fluid pressure is applied at the high-pressure radialside, the auxiliary sleeve will, as a result of the location of thetertiary seal, be urged in the axial direction, ensuring that thetertiary seal performs its required sealing function. In its simplestform, the tertiary seal is provided by face-to-face contact betweentransverse faces of the auxiliary sleeve and seal housing, when theshaft seal is in use. However, it is preferred that a separate tertiaryseal is provided, for greater sealing integrity.

[0036] Suitably, the tertiary seal comprises an O-ring located in achannel formed in the transverse end face of the auxiliary sleeve. Inaccordance with a modification, the tertiary seal comprises an O-ringlocated in a channel formed in the transverse end face of the sealhousing. Alternatively, it can comprise a spring-energised U-seal orY-seal.

[0037] Normally, the pusher sleeve will comprise a mainaxially-extending portion with a radial flange at one end in contactface-to-face with the sealing element. This design for the pusher sleeveis preferred because then the main axially-extending portion can serveto provide for accommodating the required limited axial movement of thepusher sleeve while the first sealing member maintains sealing contactwith the auxiliary sleeve, and the radial flange conveniently serves toreceive the bias force of the biasing means and apply it against thesealing element. Furthermore, the flange substantially resists radiallyinward distortion of the pusher sleeve at the one axial end of thepusher sleeve. When the pusher sleeve has this preferred form, theauxiliary sleeve preferably similarly comprises a main axially-extendingportion with a radial flange at one end facing the flange of the pushersleeve. This arrangement for the auxiliary sleeve helps to ensure thatits distortion under pressure closely conforms with that of the pushersleeve.

[0038] In another arrangement, the pusher sleeve is of the sameconstruction, but the auxiliary sleeve is in the form of a sleeve overits entire length. Then, when the shaft seal is in use, the high fluidpressure acting on the outside of the low-pressure axial end portion ofthe auxiliary seal beyond the sealing location of the first sealingmember will deflect that end portion inwardly, again reducing thelikelihood of the first sealing member being blown out of the shaftseal.

[0039] Additionally, a further secondary seal may be provided betweenadjacent radial surfaces of said sealing ring and said pusher sleeve.

[0040] The tertiary seal can be provided by a lip formed integrally onand protruding from one of said seal housing and said auxiliary sleeve,said lip sealingly contacting the other of said seal housing and saidauxiliary sleeve. Further, said secondary seal between said sealing ringand said pusher sleeve may be provided by a lip formed integrally on andprotruding from one of said pusher sleeve and said sealing ring, saidlip being maintained sealingly in contact with the other of said pusherelement and said sealing ring.

[0041] The preferred arrangements in which a lip is provided to form adirect contact seal between the pusher sleeve and the sealing ring, orbetween the auxiliary sleeve and the seal housing, respectively, avoidthe use of separate sealing members, and hence result in constructionalsimplification and lower cost.

[0042] In one preferred arrangement, the pusher sleeve comprises aninner pusher sleeve element and an outer annular pusher disc, the innerpusher sleeve element and the outer pusher disc having respective radialsurfaces maintained in contact with one another.

[0043] Alternatively, or in addition, the auxiliary sleeve comprises aninner auxiliary sleeve element and an outer annular auxiliary disc, theinner auxiliary sleeve element and the outer annular auxiliary dischaving respective radial surfaces maintained in contact with oneanother.

[0044] In this way, torsional forces acting in the region of thejunction of the axially extending portion and the forward radial portionof both the one-part pusher sleeve and the one-part auxiliary sleeve arelargely eliminated.

[0045] The shaft seal may be incorporated in a turbo-machine or otherpressurized machine, though for convenience the description whichfollows relates to the specific case of a compressor, as in the priorart examples described with reference to FIGS. 1, 1a and 2, 2 a.

[0046] For a better understanding of the invention and to show how thesame may be carried into effect, reference will now be made, by way ofexample, co the accompanying drawings in which:

[0047]FIG. 1 is a partial longitudinal sectional view through a firstknown shaft seal showing the relevant structural elements of the seal;

[0048]FIG. 1a is an enlarged view of part of the seal, showing thedistortion of certain structural elements in an exaggerated manner forillustrative purposes;

[0049]FIGS. 2, 2a are corresponding views to FIGS. 1, 1a for a modifiedknown arrangement;

[0050]FIGS. 3, 3a are corresponding views to FIGS. 1, 1a, respectively,of a first embodiment of the invention;

[0051]FIGS. 3b, 3 c respectively show, on an enlarged scale, alternativeforms of tertiary seal to that incorporated in the embodiment accordingto FIGS. 3, 3a;

[0052]FIG. 4 is an exploded view of the shaft seal according to thefirst embodiment;

[0053]FIGS. 5, 5a are corresponding views to FIGS. 3, 3a, respectively,of a second embodiment of the invention;

[0054]FIGS. 6, 6a are corresponding views to FIGS. 3, 3a respectively ofa third embodiment of the invention;

[0055]FIG. 7 is a view corresponding to FIG. 3a, showing a fourthembodiment of the invention; and

[0056]FIG. 8 is a view corresponding to FIG. 3a, showing a fifthembodiment of the invention.

[0057] The shaft seals illustrated in FIGS. 3 and 4, 5 FIG. 5, and FIGS.6, 7 and 8 are identical to that described above with reference to FIG.1, except in the respects described below. To the extent that theconstruction is the same, this is indicated by the use of identicalreference numerals

[0058] The shaft seal 2 additionally comprises an auxiliary sleeve 20disposed around the pusher sleeve 9 co-axially therewith, with a smallgap radially separating the two sleeves 9, 20 and located within arecess 17 within the radially inner flange 8 b. The auxiliary sleevecomprises a main axially-extending sleeve portion 20 a having at one enda radial flange 20 b facing the rear face of the radial flange 9 b ofthe pusher sleeve 9. The biasing springs 10 act between the flanges of 9b, 20 b of the pusher sleeve 9 and auxiliary sleeve 20, thereby urgingthe auxiliary sleeve further into the recess 17 in the seal housing B. Atertiary seal 16 provides a substantially fluid-type seal between thetransverse rear-facing end face of the sleeve portion 20 a and anadjacent forwardly-facing, transverse annular face 19 of the sealhousing. As shown, this seal is preferably in the form of a springenergised Y-seal. Alternatively, it can be an O-ring 16′ located in achannel formed in the sleeve portion 20 a (see FIG. 3b) or in theradially inward flange 8 b of the seal housing, or a spring energisedU-seal 16″ (see FIG. 3c) or a spring-energised Y-seal. The firstsecondary sealing member 12 provides a substantially fluid-tight sealbetween the auxiliary sleeve and the pusher sleeve 9.

[0059]FIG. 4 is an exploded view of the shaft seal, giving a clearindication of the geometry of the respective elements of the shaft seal.This Figure also shows the modification in which the taper shapedgrooves are formed in the sealing face of rotary sealing ring 4, ratherthan of the non-rotating sealing ring 2.

[0060] In use of the shaft seal, the high pressure fluid acting at thehigh-pressure fluid side of the primary seal acts, just as in the ceaseof the known shaft seals according to FIGS. 1, 1a and 2, 2 a, againstthe pusher sleeve 9 to cause the axially-extending sleeve portion todeflect radially inwardly. The distortion of the axially extendingsleeve portion is progressive from the axial location of secondary seal12 along the length of the pusher sleeve, because of the pressuredifferential between the inside and outside pressures acting on mainsleeve portion 20 a. The flange 9 b substantially resists distortion ofthe pusher sleeve in the region of that end. The maximum inward radialdistortion occurs at the other end.

[0061] However, in the present embodiment, as shown in FIG. 3b, the highfluid pressure acting on the auxiliary sleeve, in particular around itsexternal surface, similarly distorts the auxiliary sleeve 20. Therefore,the small gap existing between the outer surface of the pusher sleeve 9and the inner surface of the auxiliary sleeve 20 does not change much,thereby avoiding or at least minimising the possibility of thehigh-pressure acting on the secondary seal 12 from causing the seal tobe extruded into the gap. Therefore, even when operating under higherpressures e.g. upwards of 300 bar, the secondary seal 12 will not startto offer high frictional resistance to the sliding action of the pushersleeve, nor be expelled or blown out of the channel 14 in the pushersleeve 9.

[0062] It is preferred to design the auxiliary sleeve 20 such that thegap between it and the pusher sleeve 9 remains substantially constantirrespective of the pressure which is acting at the high pressure radialside. This result can be achieved by appropriate choice of the geometryand relative dimensions of the auxiliary sleeve 20 and pusher sleeve 9,and by suitable choice of the materials from which these two componentsare made. Preferably, the radial and toroidal stiffnesses of theauxiliary sleeve 20 are the same as those of the pusher sleeve 9. It isalso preferred that the materials from which the auxiliary sleeve 20 andpusher sleeve 9 are made are the same, so that the gap between those twocomponents remains substantially invariant. Irrespective of temperaturechanges.

[0063] The embodiment according to FIG. 5 shows two possiblemodifications which may be adopted singly or in combination.

[0064] The first modification, merely involves accommodating thesecondary sealing member 12 in a channel 14 formed in the auxiliarysleeve 20, rather than in the pusher sleeve 9.

[0065] The second modification is that the springs 10 do not act at oneend against the auxiliary sleeve 20, but merely function to urge thepusher sleeve 9 against the sealing ring 2. Nevertheless, it is stillconsidered with this arrangement that adequate sealing performance isprovided by the tertiary seal 16, because, when the high pressure fluidis acting at the high-pressure radial side, it will produce a net axialforce acting to urge the auxiliary sleeve 20 against the radially inwardflange 8 b of the seal housing 8, due to the locations of the secondaryseal 12 and the tertiary seal 16 on the auxiliary sleeve.

[0066] It is pointed cut that in the embodiment according to FIG. 5, theauxiliary sleeve 20 does not have any end flange as in the FIGS. 3 and 4embodiment, but is in the form of a sleeve over its entire axial extent.Because of the sealing function of the secondary seal 12, the internaland external forces acting on the axial portion of the auxiliary sleeveat the high pressure axial side of the secondary seal 12 balance eachother, whereas there is a net inward radial force acting on the portionof the auxiliary sleeve at the low-pressure axial side of the secondaryseal 12, which distorts the auxiliary seal inwardly towards the axiallyextending sleeve portion of pusher sleeve 9. This distortion producesthe effect of minimising the gap between the pusher and auxiliarysleeves on the low-pressure fluid side of seal 12, thereby minimisingthe likelihood of the seal 12 being expelled.

[0067]FIG. 6 shows another embodiment of the invention. In thismodification, no tertiary seal 16 such as shown in FIG. 3a is providedby a separate sealing member. Instead, the tertiary seal is provided bydirect sealing contact between a front face of a lip 8 c formedintegrally with the seal housing 8 and protruding axially and forwardlyfrom the radially inward flange 8 b of the seal housing 8, and the rearface of the axially extending portion 20 a of auxiliary sleeve 20.Alternatively, the lip can be formed integrally on and protruderearwardly from the axially extending portion 20 a of auxiliary sleeve20, to seal against a transverse front face of the radially inwardflange 8 b. In a similar way, the use of a secondary seal 13 in the formof a separate component as in the FIGS. 3, 3a embodiment is avoided byproviding a lip 9 c, protruding forwardly from the front face of andformed integrally with the radial flange 9 b of plusher sleeve 9 asshown in FIG. 6, the lip being in direct sealing contact with the rearface of sealing ring 2. Alternatively, the lip 9 c can be formedintegrally on the rear face of the sealing ring 2 and protruderearwardly to seal against the front face of the radial flange 9 b ofthe pusher sleeve 9. FIG. 6a shows the shaft seal under workingconditions.

[0068] A fourth embodiment of the invention shown in FIG. 7 is amodification to the pusher sleeve 9 of the third embodiment. In thismodified embodiment, the pusher sleeve, which is a single-partcomponent, is replaced with a two-part pusher sleeve, comprising aninner pusher sleeve element 23 with an axially extending portion 23 aand a radial flange 23 b at the front end of axially extending portion23 a, and an outer annular pusher disc 22, the pusher disc beingcoaxially disposed around the inner pusher sleeve 23. A front face 22′of an inner radial portion of the pusher disc 22 is maintained sealinglyin contact with the rear face 23′ of the radial flange 23 b. The pushersleeve 23 seals against the sealing ring 2 through a lip 23 c protrudingforwardly from the front face of radial flange 23 b, the lip beingformed integrally with the radial flange 23 b and in direct contact withthe rear face of sealing ring 2. Alternatively, the lip can be formedintegrally on the rear face of the sealing ring 2 and protruderearwardly to seal against a front face of radial flange 23 b.

[0069] One advantage of a two-part pusher sleeve is that torsionalforces, as encountered in the region of the junction between the radialportion 9 b and the axially extending portion 9 a of thesingle-component pusher sleeve 9, when the shaft seal is in operation,are largely eliminated.

[0070] A fifth embodiment of the invention shown in FIG. 8 is amodification to the fourth embodiment. In the fifth embodiment, theauxiliary sleeve 20, which is a single-part component, is replaced by atwo-part auxiliary sleeve comprising an inner auxiliary sleeve element24 and an outer annular auxiliary disc 25. The outer annular auxiliarydisc is coaxially disposed around the inner auxiliary sleeve element 24at its forward end, and the inner auxiliary sleeve element and the outerannular auxiliary disc have respective front and rear radial surfaces24′ and 25′ sealingly maintained in contact with one another.

[0071] In another embodiment, the pusher sleeve is of one-partconstruction (such as described with reference to FIGS. 6, 6a) and theauxiliary sleeve is of two-part construction (such as described withreference to FIG. 8).

[0072] As in the case of the two-part pusher sleeve of the fourthembodiment, an advantage of replacing the single-component auxiliarysleeve with a two-part auxiliary sleeve is that torsional forces, asencountered in the region of the junction between the radial flange 20 band the axially extending portion 20 a of the single-component auxiliarysleeve 20 when the shaft seal is in operation, are largely eliminated.Also, when in operation, the two-part auxiliary sleeve deforms in acompliant manner with the deformation of the pusher sleeve.

[0073] Under normal operating conditions as shown in FIGS. 6a, 7 and 8,the force acting on the lip seals 8 c, 23 c due to the very high fluidpressure acting within the compressor is very large due to therelatively small contact area of the lip seals, and thus a substantiallyfluid-tight seal is maintained.

[0074] The lip seal arrangements used in the embodiments shown in FIGS.6, 6a, 7 and 8 avoid the use of separate sealing members serving as thesecondary seal 13 and the tertiary seal 16 according to the embodimentshown in FIGS. 3, 3a. This results in a constructional simplificationand hence lower cost.

[0075] As an alternative to the biasing spring 10, a wave spring forexample in the form of a single annulus of suitable sheet material, e.g.metal, (or several stacked together) may be deformed so as to formsuccessive undulations at different angular positions about the axis ofthe annulus. The deformed annulus is compressed between the primarysealing ring 2 and the auxiliary sleeve 20 or radially inward flange 8 b(as the case may be), thereby providing the required biasing action inthe manner of a leaf spring.

[0076] In the described embodiments, the source of the high-pressurefluid is the working fluid of the compressor, whose pressure accordinglyincreases with increasing compressor operating speed. Where a separatesource of high-pressure fluid from the working fluid is used, itspressure will normally be held at a given delivery pressure. When thecompressor is at rest, the net force acting on the primary seal ispreferably a closing force, maintaining the sealing ring 2 against thesealing ring 4. However, when the compressor has speeded upsufficiently, the separating force generated by the tapered grooves orrecesses in the one sealing ring or the other of the primary seal issufficient to separate the two rings. Therefore, the operation isessentially the same as in the case where the working fluid of thecompressor is the source of the high-pressure fluid. Although it ispreferred in this embodiment that the sealing ring 2 is held against thesealing ring 4 when the compressor is at rest, it is possible for theshaft seal to be slightly open under rest conditions, since theessential requirement is merely that the shaft seal provides contactlessoperation when the compressor is operating at normal operational speed.

1. A shaft seal comprising a sealing element (2), a rotary sealing part(4) mounted coaxially with the sealing element and forming therewith acontactless primary seal between opposed faces of the sealing element(2) and rotary sealing part (4) to substantially prevent fluid flowacross the primary seal from a high pressure radial side to alow-pressure radial side, a seal housing (8), a pusher sleeve (9)disposed, between the seal housing and the sealing element, coaxiallywith and in contact with the sealing element (2), biasing means (10)urging the pusher sleeve (9) away from the seal housing (8) and againstthe sealing element (2) to urge the sealing element axially towards therotary sealing part (4), and a first sealing member (12) disposed aboutthe pusher sleeve (9) and located, in a channel (14), in communicationwith the high-pressure radial side to provide a secondary seal for thepusher sleeve (9) between the high-pressure and low-pressure radialsides, characterised by an auxiliary sleeve (20) disposed around thepusher sleeve (9) coaxially therewith and maintained in sealing contactwith the pusher sleeve (9) by the first sealing member (12), theauxiliary sleeve being arranged to be urged in an axial direction byfluid pressure acting at the high-pressure radial side to form atertiary seal (16) between the auxiliary sleeve (20) and seal housing(8).
 2. A shaft seal according to claim 1, wherein said channel (14) inwhich said first sealing member (12) is located is formed in the pushersleeve (9).
 3. A shaft seal according to claim 1, wherein said channel(14) in which said first sealing member (12) is located is formed in theauxiliary sleeve (20).
 4. A shaft seal according to any preceding claim,wherein the biasing means (10) acts between the pusher sleeve (9) andthe auxiliary sleeve (20).
 5. A shaft seal according to any one ofclaims 1 to 3, wherein the biasing means (10) acts between the sealhousing (8) and the pusher sleeve (9).
 6. A shaft seal according to anypreceding claim, wherein the tertiary seal (16) is provided byface-to-face contact between transverse faces of the auxiliary sleeveand seal housing, when the shaft seal is in use.
 7. A shaft sealaccording to any one of claims 1 to 5, wherein the tertiary seal (16)comprises an O-ring (17) located in a channel (18) formed in thetransverse end face of the auxiliary sleeve (9).
 8. A shaft sealaccording to any one of claims 1 to 5, wherein the tertiary seal (16)comprises an O-ring (17) located in a channel (18) formed in thetransverse end face of the seal housing (8).
 9. A shaft seal accordingto any one of claims 1 to 5, wherein the tertiary seal (16) comprises aspring-energised U-seal.
 10. A shaft seal according to any one ofclaims, 1 to 5, wherein the tertiary seal (16) is a spring-energisedY-seal.
 11. A shaft seal according to any preceding claim, wherein thepusher sleeve (9) comprises a main axially extending portion (9 a) witha radial flange (9 b) at one end in face-to-face contact with thesealing element (2) and the auxiliary sleeve (20) also comprises a mainaxially extending portion with a radial flange (20 b) at one end facingthe flange (9 a) of the pusher sleeve (9).
 12. A shaft seal according toany one of claims 1 to 10, wherein the pusher sleeve (9) comprises amain axially extending portion (9 a) with a radial flange (9 b) at oneend in face-to-face contact with the sealing element (2) and theauxiliary sleeve (20) is in the form of a sleeve over its entire length.13. A shaft seal according to any preceding claim, additionallycomprising a further secondary seal (13) between adjacent radialsurfaces of said sealing ring (2) and said pusher sleeve (9).
 14. Ashaft seal according to any one of claims 1 to 4, wherein said tertiaryseal (16) is provided by a lip (8 c) formed integrally on and protrudingfrom one of said seal housing (8) and said auxiliary sleeve (20), saidlip sealingly contacting the other of said seal housing and saidauxiliary sleeve.
 15. A shaft seal according to claim 13 as appended toany one of claims 1 to 11 or according to claim 14, wherein saidsecondary seal (13) between said sealing ring (2) and said pusher sleeve(9) is provided by a lip (9 c) formed integrally on and protruding fromone of said pusher sleeve (9) and said sealing ring (2), said lip beingmaintained sealingly in contact with the other of said pusher elementand said sealing ring.
 16. A shaft seal according to claim 14 or 15,wherein said pusher sleeve (9) comprises an inner pusher sleeve element(23) and an outer annular pusher disc (22), said inner pusher sleeveelement and said outer annular pusher disc having respective radialsurfaces (23′, 22′) maintained in contact with one another.
 17. A shaftseal according to claim 14, 15 or 16, wherein said auxiliary sleeve (20)comprises an inner auxiliary sleeve element (24) and an outer annularauxiliary disc (25), said inner auxiliary sleeve element and said outerannular auxiliary disc having respective radial surfaces (24′, 25′)maintained in contact with one another.
 18. A turbo-machine or otherpressurized machine incorporating a shaft seal according to anypreceding claim.